Shares of Suncor and Canadian Natural Resources plunged on environmental fears and rising U.S. oil production. Why proposed pipelines, like Keystone XL, could help pump them back up.

Investors once paid premium prices for marquee Canadian energy stocks like Suncor Energy and Canadian Natural Resources because they operate in a politically stable country and boast a combination of long-lived reserves and healthy growth outlooks.

Nothing has changed on those counts, but Suncor (ticker: SU) and Canadian Natural Resources (CNQ) have fallen out of favor over the last few years as strong growth in U.S. oil and gas...